A HVDC system comprises a first and second converter station each containing a voltage source converter (VSC) for transferring electric power from a first alternating current (AC) network to a second AC network.
Voltage source converters (VSC) are not only used in high voltage direct current (HVDC) systems, but also for example as Static Var Compensators (SVC). In the HVDC application, the voltage source converter is connected between a direct current (DC) link and an AC network, and in the application as Static Var Compensator, the voltage source converter is connected between a direct voltage source and an AC network. In both these applications, the voltage source converter must be able to generate an AC voltage of the same frequency as that of the AC network. The reactive and the active power flow through the converter are controlled by modulating the amplitude and the phase position, respectively, of the AC voltage generated by the voltage source converter in relation to the voltage of the AC network.
In particular the voltage source converter equipped with series-connected transistors (IGBT) has made it possible to use this type of converter for comparatively high voltages. A pulse width modulation (PWM) is used for control of the generated AC voltage which enables a very fast control of the voltage.
From U.S. Pat. No. 6,400,585, which hereby is incorporated by reference, a control system for voltage control of a converter station in a HVDC system is previously known. The object of the control system is to maintain the voltage of a direct current link within safe operation limits also at abnormal voltage conditions.
The known HVDC system comprises a first and a second converter station each having a voltage source converter connected between a DC link and an AC network on each side of the DC link. A current control system for the converter station has means for control of active power flow between the DC link and the AC network by influencing the phase displacement between the bus voltage in the AC network and the bridge voltage of the voltage source converter. The terms bus voltage and bridge voltage are explained further below. The control system comprises means for generation of a phase change order signal in response to an indication of an abnormal voltage condition at the DC link, and means for influencing the phase position of the bridge voltage in response to said phase change order signal, so as to ensure that the phase displacement between the bridge voltage and the bus voltage will result in an active power flow from the DC link to the AC network. A phase-locked loop means (PLL) ensures that the control system of the converter station works in synchronism with the phase position of the bus voltage of the AC network.
The active power flow into the DC link must be balanced. This means that the active power leaving the link must be equal to the power received by the link Any difference may cause the DC voltage to rapidly increase or decrease. To achieve this power balance one of the converter stations controls the DC voltage. The other converter station thus may control the active power flow of the DC link by controlling the DC current accordingly. Commonly the upstream converter station controls the DC voltage while the downstream converter controls the active power flow.
Restoring power after a wide-area power outage in an AC network or AC grid can be difficult. A plurality of power stations needs to be brought back on-line. Normally, this is done with the help of power from the rest of the grid. In the absence of grid power, a so-called black start needs to be performed to bootstrap the power grid into operation.
To provide a black start, some power stations are typically equipped with small diesel generators which can be used to start larger generators of several megawatts capacity, which in turn can be used to start the main power station generators. Generating plants using steam turbines require station service power of up to 10% of their capacity for boiler feedwater pumps, boiler forced-draft combustion air blowers, and for fuel preparation. It is, however, uneconomic to provide such a large standby capacity at each station, so black-start power must be provided over the electrical transmission network from other stations.
A typical black start sequence based on a real scenario might be as follows:                A battery starts a small diesel generator installed in a hydroelectric generating station.        The power from the diesel generator is used to bring the hydroelectric generating station into operation.        Key transmission lines between the hydro station and other areas are energized.        The power from the hydro dam is used to start one of the coal-fired base load plants.        The power from the base load plant is used to restart all of the other power plants in the system including the nuclear power plants.        Power is finally re-applied to the general electricity distribution network and sent to the consumers.        
To restore the power after an outage is not an easy process. Small disturbances are continually occurring while the system is weak and fragile during the restoration process, and the grid will experience different conditions, from a dead network over a variety of weak network conditions to a normal strong AC network. In order to maintain the frequency and voltage stability during the restoring process, an overall coordinated system restoration plan is necessary.
When a converter is connected to an island network with only generation, for instance a windfarm, or only consumption, or the mixture of both, it will be very difficult to predict the active power and reactive power. It will thus be difficult to determine a desired active power Pref and a desired reactive power Qref and it will be unpractical to control them.
When a converter is connected to a dead electric AC network, i.e., no power supply at all, the above described known control system will fail to work, because there is no AC voltage to synchronize for the PLL, and the current control will not work as the current is determined naturally by the connected load.
When a converter is connected to a very weak electric AC network, i.e., the existing short circuit power in the network is approximately equal to or less than the converter rating, it is very difficult for the above described known control system to maintain the stability, as the weak AC network gives a more oscillating AC bus voltage, which leads to the oscillating of the PLL and current control as both systems use the AC bus voltage as input.